Emergency services have taken part in a response exercise at the Dow silicone works in Barry.
The plant's own emergency response team led the drill, involving police and other authorities rehearsing their response if an incident were to happen.
In June, a chemical release at the site led to nearby residents being told to shut their windows and doors as a precaution after a misty white cloud emerged.
The leak was later traced to a faulty pump.
Inspector Richard Lloyd from Barry police station said: "Training exercises take place regularly to ensure that the emergency services are prepared to respond together."
"They are are not intended to cause alarm and should be a source of reassurance to the community that we have plans in place help keep them safe."